The present invention relates to a method for extracting a gas from a gas mixture that contains the gas.
Such a method is already known in the sector of the purification of untreated biogas, which for example comes from the anaerobic digestion of waste sludge, organic waste, manure, and similar. This untreated biogas primarily consists of methane and typically 30% to 50% CO2 gas.
In order to be able to be used as a natural gas replacement fuel and injected into the natural gas network, this untreated biogas must be purged of the CO2 as much as necessary. The thus purified gas is also called green gas or biomethane because upon combustion it does not contribute to global warming, in contrast to what happens with the combustion of fossil fuels.
A method is already known by which the untreated biogas is purified by bringing it into contact with a sorption medium in the form of an absorption liquid that has the property of absorbing the CO2 gas, after which the purified biogas and the liquid sorption medium with the CO2 sorbed therein can be removed separately.
According to another known method for extracting CO2 from a gas mixture, use is made of a venturi ejector to bring the gas mixture to be purified in contact with a liquid sorption medium that is able to sorb the gas to be extracted from the gas mixture upon contact with the gas mixture in the venturi ejector.
With this known method it is also known that the liquid sorption medium is regenerated by extracting the sorbed CO2 from it so that the sorption medium can be used again to purify untreated biogas.
The regeneration is done by heating and requires a lot of energy as the quantity of heat energy needed to regenerate it is proportional to the quantity of CO2 that must be removed.
This heat can be partially recovered for low temperature applications if necessary on location, but the heat consumption nevertheless remains a disadvantage.
Other known techniques such as washing out with water, PSA (Pressure Swing Absorption, TSA (Temperature Swing Absorption) and the use of selective membranes require less heat energy, but then require a lot of compressor power and thus a lot of energy for driving the compressors. These last techniques also remove CO2 from the untreated biogas less efficiently, such that the quality of the green biomethane supplied is lower and more methane is also lost, such that a post-treatment of the extracted CO2 gas, which still contains methane, is necessary. This post-treatment is necessary because methane emissions contribute to global warming.
The invention not only relates to the purification of untreated biogas, but more generally to the extraction of a certain gas from a gas mixture.
Another typical example of this is the drying of moist air or a moist gas, whereby water vapour has to be extracted from the air.
Dryers are already known that are used for this purpose that operate according to the principle of cold drying or which make use of an absorbing mass through which the air to be dried or gas to be dried is driven.
These known techniques are voluminous and cause a pressure loss of the gas. As such a gas then has to be compressed such a pressure loss results in substantially higher compression energy.